I am using Fluxbox, that said I am not that much of a gamer.
I don't think I get any performance boost with fluxbox though. It's just that I find it way more eye-candy than anything else (once properly configured!)

I was using KDE before Fluxbox. It's a great desktop/window manager. I just find it a bit less eye candy than fluxbox and too "heavy".
If you have a good machine, I doubt it will have any impact on your performances though.